If you make the update by yourself you always take the risk to destroy your nav hardware. That's the reason, why the dealer should make the updates.
I know some cases where the dealer shoot the nav computer, too. Then you'll get a new one.

In germany we took wounded mkIV's to VDO, to reset them. It's not cheap (~90$) and took a lot of time (~30 days), but there's no other known way this time.